Since we established a correlation between tumorigenicity and the ability to synthesize plasminogen activator in one system and since we obtained antibodies against the plasminogen activator which inhibits its enzymatic activity, we propose the following: a. Localization of the intracellular plasminogen activator and/or its precursor. b. Studying the effect of specific inhibition of plasminogen activator on the transformation process as well as on tumorigenicity. c. Determining the effect of anti-plasminogen activator on the tumorigenicity of enzyme-secreting cells. d. Purifying plasminogen activator by use of immobilized antibodies. e. Establishing the differences between the plasminogen activator(s) secreted by chemical and Rous sarcoma transformants of hamster cells, the plasminogen activators produced by normal hamster kidney cells and our purified plasminogen activator, ( the enzyme from kidney has an immunological identity with our purified enzyme even though its ability to convert plasminogen to plasmin is unaffected by specific antibodies). f. Characterizing the structure of hamster-derived plasminogen activator in greater detail. g. Purifying and characterizing plasminogen activator produced by a human cell line.